Non-invasive imaging and analysis of targets is a valuable technique for acquiring information about systems or targets without undesirable side effects, such as damaging the target or system being analyzed. In the case of analyzing living entities, such as human tissue, undesirable side effects of invasive analysis include the risk of infection along with pain and discomfort associated with the invasive process. In the case of quality control, it enables non-destructive imaging and analysis on a routine basis.
Optical coherence tomography (OCT) is a technology for non-invasive imaging and analysis. OCT typically uses a broadband optical source, such as a super-luminescent diode (SLD), to probe and analyze or image a target. It does so by applying probe radiation from the optical source to the target and interferometrically combining back-scattered probe radiation from the target with reference radiation also derived from the optical source.
The typical OCT optical output beam has a broad bandwidth and short coherence length. The OCT technique involves splitting the output beam into probe and reference beams, typically by means of a beam-splitter, such as a pellicle, a beam-splitter cube or a fiber coupler. The probe beam is applied to the system to be analyzed (the target). Light or radiation is scattered by the target, some of which is back scattered to form a back-scattered probe beam, herein referred to as signal radiation.
The reference beam is typically reflected back to the beam-splitter by a mirror. Light scattered back from the target is combined with the reference beam, also referred to as reference radiation, by the beam-splitter to form co-propagating reference radiation and signal radiation. Because of the short coherence length only light that is scattered from a depth within the target whose optical path length is substantially equal to the path length to the reference mirror can generate a meaningful interferometric signal.
Thus the interferometric signal provides a measurement of scattering properties at a particular depth within the target. In a conventional time domain OCT system, a measurement of the scattering values at various depths can be determined by varying the magnitude of the reference path length, typically by moving the reference mirror. In this manner the scattering value as a function of depth can be determined, i.e. the target can be scanned.
There are various techniques for varying the magnitude of the reference path length. Electro-mechanical voice coil actuators can have considerable scanning range, however, there are problems with maintaining the stability or pointing accuracy of the mirror. Fiber based systems use fiber stretchers, however, they have speed limitations and have size and polarization issues. Rotating diffraction gratings can run at higher speeds, however, are alignment sensitive and have size issues.
Piezo devices can achieve high speed scanning and can have high pointing accuracy, however to achieve a large scanning range requires expensive controls systems and have limited speed. A scanning method that effectively amplifies the scan range of a piezo device is described in U.S. Pat. Nos. 7,526,329 and 7,751,862 incorporated herein.
The technique described in these incorporated references uses multiple reference signals with increasing scan range and correspondingly increasing frequency interference signals. This scanning method can achieve large scan range at high speed with good pointing stability.
In swept source Fourier domain OCT systems depth scanning is accomplished by repeatedly sweeping the wavelength of the optical source. The wavelength range over which the optical source is swept determines the depth resolution. The period of the sweep repetition rate determines the period of the depth scans.
In addition to depth scanning, lateral scanning is required for many imaging and analysis applications. There are many conventional techniques for lateral scanning, such as the use of stepper or linear motors. Some applications require angular scanning, which is typically accomplished by electro-mechanical oscillating mirrors, typically referred to as salvo-scanners.
An example of an imaging application that requires angular scanning is the ophthalmic application of imaging the retina of the eye. The lens of the eye focuses collimated light on the retina. As a consequence lateral scanning by stepper or linear motors is not and effective retinal scanning technique. An angular scanning technique is required.
Conventional salvo-scanners are physically bulky and are inefficient because of their oscillating nature. They are therefore not suitable for compact or portable devices. There is therefore an unmet need for a method, apparatus and system for angular scanning over an extended depth range in a compact and power efficient manner.